Most modern wind turbines are controlled and regulated continuously during operation with the purpose of ensuring optimal performance of the wind turbines in all operating conditions, such as at different wind speeds or subject to different demands from the power grid. Desirably, the wind turbine can also be regulated to account for fast local variations in the wind velocity—the so-called wind gusts. Also, as the loads on each of the blades vary due to e.g. the passing of the tower or the actual wind velocity varying with the distance to the ground (the wind profile), the ability to regulate each of the wind turbine blades individually advantageously enables the loads to be balanced, reducing the yaw and tilt of the rotor.
A well-known and effective method of regulating the loads on the rotor is by pitching the blades. However, with the increasingly longer blades on modern wind turbines (which at present can be of 60 m or longer) pitching becomes a relatively slow regulation method incapable of changing the blade positions fast enough to account for e.g. wind gusts or other relatively fast load variations.
Another way of regulating the blades is by changing their aerodynamic surfaces or shapes over parts or the entire length of the blade, thereby increasing or decreasing the blade lift or drag correspondingly. Different means of changing the airfoil shape are known such as active trailing edges, or different types of movable or adjustable flaps (e.g. trailing edge flaps, leading edge slats or Krueger flaps, ailerons, or stall inducing flaps). Such different means are here and in the following referred to in common as aerodynamic devices or devices for modifying the aerodynamic surface or shape of the blade. One important advantage of the relatively small aerodynamic devices is a potentially faster response due to less inertia than if the whole blade is being pitched.
In order to obtain the desired regulation of the blades, the flaps or active trailing edges often extend over considerable lengths of the blade and are traditionally actuated or moved in a chordwise plane transverse to the length of the blade. However, known systems may exhibit considerable power consumption during operation and problems with their operational speed. Moreover, known systems are generally exposed to hard wear and corresponding relative short lifetime and durability.